Conventionally, a C2-C20 fluorine-containing organic acid is known and this fluorine-containing organic acid, in particular a C2-C8 fluorine-containing organic acid, is used as an emulsifier, for example, for producing polymers by emulsion polymerization. In an emulsifier, impurities such as water and others may be mixed depending on a production method and/or a manner of use of an emulsifier. In order not to have unintended effect on emulsion polymerization, it is preferable that an amount of impurities in an emulsifier is small. In addition, emulsifiers are relatively expensive, so it is desirable to collect them after use and reuse them.
For example, Patent Literature 1 discloses that a mixture containing a fluorocarboxylic acid, which is one of emulsifiers, and water is dehydrated by contacting it with a concentrated sulfuric acid to obtain a dehydrated fluorocarboxylic acid solution, and then the dehydrated fluorocarboxylic acid solution is purified by distillation operation.